Almost all cellular functions are modulated by a variety of different intracellular messengers. This proposal focuses on the regulation and function of one of these messengers, cyclic GMP (cGMP). The balance of its synthesis and breakdown regulates the level of cGMP within a cell. The synthesis of cGMP is controlled by the enzyme guanylyl cyclase and until recently two main classes of guanylyl cyclase were known, receptor and soluble guanylyl cyclases. Receptor guanylyl cyclases are integral membrane proteins that are regulated by either extracellular peptide hormones or intracellular calcium-binding proteins. Guanylyl cyclases of this class regulate the cGMP levels in the kidney and in the photoreceptors of the retina. Soluble guanylyl cyclases are located in the cytoplasm and have classically been regulated by the gaseous messenger, nitric oxide. They are particularly important in regulating blood pressure via vascular smooth muscle tone. An example of the importance of cGMP in human health and disease is shown by the widespread use of the drug, sildenafil (Viagra) that elevates cGMP levels by inhibiting an enzyme that normally breaks down cGMP. Using an insect, Manduca sexta, as a model organism my lab has identified two novel classes of guanylyl cyclase. The first is related to receptor guanylyl cyclases but is not an integral membrane protein. The second is related to soluble guanylyl cyclases but is nitric oxide insensitive. We have evidence to suggest that this nitric oxide-insensitive soluble guanylyl cyclase mediates the action of a neuropeptide that regulates a specific behavioral program and associated physiological changes in Manduca and another insect, Drosophila melanogaster. The major focus of this proposal is to test this hypothesis directly combining the use of cellular and molecular approaches in Manduca with a genetic approach in Drosophila. In addition, we will use molecular and biochemical techniques to determine the biochemical cascade involved in the activation of this novel guanylyl cyclase. Members of these novel classes of guanylyl cyclase are also present in another model organism, Caenorhabditis elegans. Most signal transduction cascades are highly evolutionarily conserved and it is likely that examples of these guanylyl cyclases will be identified in mammals. A more detailed knowledge of the variety of mechanisms that regulate the levels of cGMP in different tissues will undoubtedly have a significant impact basic cellular physiology in a wide variety of systems.